The present invention relates to the field of solid oxide fuel cells, and more particularly, relates to a method of making a solid oxide fuel cell having controlled porosity in the anode, cathode, electrolyte and interconnect.
A fuel cell is a device in which a first reactant, a fuel such as hydrogen or a hydrocarbon, is electrochemically reacted with a second reactant, an oxidant such as air or oxygen, to produce a DC electrical output. A fuel cell includes an anode, or fuel electrode, which enhances the rate at which electrochemical reactions occur on the fuel side. There is also a cathode, or oxidant electrode, which functions similarly on the oxidant side. In the solid oxide fuel cell (hereafter SOFC), a solid electrolyte, made of, for example, dense yttria-stabilized zirconica (YSZ) ceramic separates a porous ceramic anode from a porous ceramic cathode. The anode is made of, for example, nickelous oxide/YSZ cermet, and the cathode is made of, for example, doped lanthanum manganite.
In such an SOFC, the fuel flowing to the anode reacts with oxide ions to produce electrons and water, which is removed in the fuel flow stream. The oxygen reacts with the electrons on the cathode surface to form oxide ions that diffuse through the electrolyte to the anode. The electrons flow from the anode through an external circuit and thence to the cathode. The electrolyte is a nonmetallic ceramic that is typically a poor or nonconductor of electrons, ensuring that the electrons must pass through the external circuit to do useful work. However, the electrolyte permits the oxide ions to pass through from the cathode to the anode.
Each individual electrochemical cell, made of a single anode, a single electrolyte, and a single cathode, generates a relatively small voltage. To achieve higher voltages that are practically useful, the individual electrochemical cells are connected together in series to form a stack. The cells are connected in series electrically in the stack. The fuel cell stack includes an electrical interconnect between the cathode and the anode of adjacent cells. The fuel cell assembly also includes ducts or manifolding to conduct the fuel, oxidant and reactant products into and out of the stack.
Numerous publications describe conventional SOFC which completely oxidize methane to carbon dioxide and water. These SOFC are not designed to conduct chemical processes, but rather to generate electricity from fuel gas and air (or oxygen). The processes conducted within SOFC are selected for essentially complete combustion rather than partial combustion and require completion of an external electric circuit or oxidation of fuel gas for continuous operation.
The typical SOFC comprises an anode made of a mixture of nickel metal and YSZ and runs at 800-1000.degree. C. since internal reforming is most efficient at these high temperatures. The ideal fuel for the anode is hydrogen but dangers of flammability, storage and energy storage density complicate its use. More commonly, the fuels used can be light hydrocarbons such as methane, propane, ethanol and methanol. Heavier fuels such as JP8 (jet fuel) and kerosene can also be used but in some cases the internal reforming is not efficient enough to reform the fuel and carbonaceous products are built up in the anode. Water vapor is typically added to the fuel source to aid reforming.
At the startup of a SOFC, the temperature is low and there exists the potential for inefficient reforming to create carbonaceous residue that can clog the pores of the anode and reduce efficiency of the SOFC.
Also, the trend in SOFC evolution or development is to lower the operating temperature of the SOFC to 550-800.degree. C. so that less exotic materials can be used for interconnects, electrical connections and materials of construction for the housing of the SOFC. In lowering the temperature, the efficiency of the SOFC is also decreased, thereby leading to incomplete reforming of the fuel and consequent buildup of carbon and carbonaceous products in the anode. This problem is compounded when heavier fuels are utilized.
Solutions have been proposed by others to reduce carbon buildup in situations other than SOFC operation. For example, Mazanec et al. U.S. Pat. No. 5,306,411, the disclosure of which is incorporated by reference herein, have proposed using a catalyst in a cylindrical shell to dehydrogenate hydrocarbons to form hydrogen gas. The shell is partially filled with a bed of catalytic ceramic or metallic materials, including rhodium, ruthenium, palladium and platinum.
Brownlow et al. U.S. Pat. No. 4,474,731, the disclosure of which is incorporated by reference herein, discloses the addition of nickel or palladium as a pyrolysis catalyst to assist in the removal of carbonaceous residues resulting from the pyrolysis of the organic binder from a ceramic greensheet in a low oxygen atmosphere.
Cowan, Jr. et al. U.S. Pat. No. 4,778,549, the disclosure of which is incorporated by reference herein, discloses the addition of ruthenium, rhodium, palladium, osmium, iridium or platinum as a pyrolysis catalyst to assist in the removal of carbonaceous residues resulting from the pyrolysis of the organic binder from a ceramic greensheet in a neutral atmosphere.
Fisher et al. U.S. Pat. No. 5,246,791, the disclosure of which is incorporated by reference herein, discloses a reforming catalyst for a molten carbonate fuel cell. The catalyst may be rhodium, ruthenium, platinum or gold, either of which may be used in conjunction with nickel.
It would be desirable to have an SOFC with a relatively low operating temperature and high reforming efficiency for many fuel sources that would include light and medium hydrocarbons.
Accordingly, it is a purpose of the present invention to have an improved SOFC having a catalytic anode for greater efficiency in reforming fuels without carbonaceous residues.
It is a further purpose of the present invention to have an improved SOFC suitable for operating at a relatively low temperature and having a high reforming efficiency.